2510332 - February 2009
Incident rays closer to the mirror-tilt angle contribute the bulk of scattered light into the collection aperture. The case of Figure 10 represents a telecentric f/3 system for an older design 10 degree device with a projection axis along the 20-degree elevation angle (i.e., nominal 20-degree illumination angle). For telecentric systems, the illumination angle nominally is 2X the mirror tilt angle, and the numerical aperture typically is set by the mirror- tilt angle. So, in Figure 10, the 20-degree elevation angle is along the nominal projection axis, and the projection cone (numerical aperture) is ±10 degrees from it. This maximizes pupil fill and aligns the illumination pupil to be nominally centered in the projection pupil for maximum throughput (lumens) with good contrast. However, a large percentage of the scattered light can be avoided by shifting the illumination angle to higher angles, keeping the same numerical aperture (±10 degrees in this case). The high scattered-light content of the shallow angles then is avoided for much improved contrast. However, the illumination pupil now is misaligned with the projection pupil by the amount of the increase, causing lower lumens. For a typical 10-degree telecentric system and lamp-pupil profile, the tradeoff between lumens and contrast by increasing illumination angle is shown in Figure 11. The device used in this example is now obsolete, but the behavior illustrated is representative of current device behavior.
22 23 24 Illumination Angle - degrees
Figure 11. Contrast and Lumens as a Function of Illumination Angle for F/3 Telecentric System (17-µm, 10-degree tilt, 0.8-µm mirror-gap device)
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